Methods of fabricating silicon-on-insulator (SOI) semiconductor devices using blanket fusion bonding

ABSTRACT

A method for fabricating silicon-on-insulator (SOI) semiconductor devices, wherein the piezoresistive pattern is defined within a blanket doped layer after fusion bonding. This new method of fabricating SOI semiconductor devices is more suitable for simpler large scale fabrication as it provides the flexibility to select the device pattern/type at the latest stages of fabrication.

TECHNICAL FIELD

The present invention relates generally to semiconductor manufacturing techniques and semiconductor devices, and more particularly to silicon-on-insulator (SOI) device manufacturing methods and devices.

BACKGROUND

Current methods for fabricating silicon-on-insulator (SOI) semiconductor devices involve a complex fusion bonding process that includes many fabrication steps prior to the actual fusion bonding. Some of these steps include masking, deposition, and etching. More specifically, current SOI fabrication methods require that a piezoresistive pattern be defined within the semiconductor substrate before fusion bonding. Patterning before fusion bonding, however, limits fabrication design as the final piezoresistive design must be selected during the early processes of fabrication, which results in less than ideal yields and increased wafer processing costs.

For illustrative examples, reference is made to U.S. Pat. No. 5,286,671, entitled “FUSION BONDING TECHNIQUE FOR USE IN FABRICATING SEMICONDUCTOR DEVICES”, issued Feb. 15, 1994 and U.S. Pat. No. 7,439,159, entitled “FUSION BONDING PROCESS AND STRUCTURE FOR FABRICATING SILICON-ON-INSULATOR (SOI) SEMICONDUCTOR DEVICES”, issued Oct. 21, 2008, to A. D. Kurtz et. al., both of which assigned to the assignee hereof, Kulite Semiconductor Products, Inc. The entire disclosures of which are hereby incorporated by reference as if being set forth in their entirety herein.

The present invention describes an improved fusion bonding process for fabricating SOI semiconductor devices, wherein the piezoresistive pattern may be defined after fusion bonding, which therefore provides more versatility in SOI fabrication and cheaper manufacturing costs.

BRIEF SUMMARY

Embodiments of the present invention provide a method for fabricating a silicon-on-insulator device, comprising doping a first surface of a first semiconductor wafer to produce a first doped layer, having a first doped side and a second doped side, and a semiconductor base layer; oxidizing the first side of the first doped layer; bonding the oxidized first doped layer to an oxidized second semiconductor wafer; removing the semiconductor base layer to expose the second doped side of the first doped layer; and patterning the second doped side of the first doped layer.

Other embodiments provide a method for fabricating a silicon-on-insulator device, comprising bonding a first semiconductor wafer, having a base layer, a first doped layer disposed on the base layer, and a first oxidized layer disposed on the first doped layer, to a second semiconductor wafer having a second oxidized layer disposed thereon, wherein the bond is between the first oxidized layer and the second oxidized layer; removing the base layer of the first semiconductor wafer to expose the first doped layer; and patterning the first doped layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a silicon “pattern” wafer having a doped layer and an oxidized layer disposed thereon.

FIG. 2 illustrates an example silicon “substrate” wafer having an oxidized top surface.

FIG. 3 illustrates the pattern wafer of FIG. 1 bonded to the substrate wafer of FIG. 2.

FIG. 4 illustrates an example silicon-on-insulator (SOI) semiconductor device in accordance with the present invention.

FIG. 5 illustrates the SOI of FIG. 4 having a piezoresistive pattern etched into a doped layer in accordance with the present invention.

FIG. 6 illustrates a method flow diagram further detailing example methods of SOI fabrication.

DETAILED DESCRIPTION

Although many embodiments of the invention are explained in detail, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the invention is limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the preferred embodiments, specific terminology will be resorted to for the sake of clarity.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

Also, in describing the many embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

By “comprising” or “containing” or “including” is meant that at least the named compound, element, particle, or method step is present in the composition or article or method, but does not exclude the presence of other compounds, materials, particles, method steps, even if the other such compounds, material, particles, method steps have the same function as what is named.

It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Similarly, it is also to be understood that the mention of one or more components in a device or system does not preclude the presence of additional components or intervening components between those components expressly identified.

Referring now to the drawings, in which like numerals represent like elements, exemplary embodiments of the present invention are herein described. It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in typical pressure sensor assemblies and chip-package assemblies and methods of making and using the same. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein.

Embodiments of the present invention provide methods for fabricating silicon-on-insulator (SOI) semiconductor devices, wherein the piezoresistive pattern is defined within a blanket doped layer after fusion bonding. This new method of fabricating SOI semiconductor devices is more suitable for simpler, large scale fabrication as it provides the flexibility to select the device pattern/type at later stages of fabrication, therefore distinguishing this method over prior art methods.

In the methods described herein, two separate semiconductors wafers are first prepared independently. Referring to FIG. 1, there is shown a first semiconductor wafer 100, which may also be referred to as the “pattern wafer” 100. The first semiconductor wafer 100 comprises a first surface 103, and a second surface 107. The pattern wafer 100 may be made of many materials, for example but not limited to, silicon. The pattern wafer 100 may be composed of N-type silicon, and in example embodiments, may have a single-crystalline structure. It shall be understood that the pattern wafer 100 may be of many geometries, and more specifically, may have, for example, a circular, square, or rectangular cross-section, or may take the form of a commercially available silicon wafer. Further, the pattern wafer 100 may undergo pre-processing steps, for example but not limited to, cleaning, etching, polishing, and/or lapping to provide a smooth surface finish before additional processing.

A first layer of oxide 115 may be grown on the pattern wafer 100. In an example embodiment, the first oxide layer 115 may be about 200 angstroms thick or thinner. One skilled in the art will appreciate that the growth of oxide layers on silicon substrates is well known in the art. For example, the first oxide layer 115 may be grown using thermal oxidation of the first surface 103 of the pattern wafer 100. By way of further example, such a thermal oxidation process may include heating the pattern wafer 100 to a temperature between about 1000° C. and 1300° C., and passing oxygen over the first surface 103 of the pattern wafer 100.

The pattern wafer may be blanket doped through the first oxide layer 115 to create a uniform doped layer 110 under the first surface 103 and a base layer 105 in the rest of the pattern wafer. In an example method, the pattern wafer is P++ boron doped, however other doping materials may also be used.

By utilizing the first oxide layer 115, “degenerate” doping can be achieved under surface 103, while preserving the requisite smoothness of the pattern wafer surface, analogously to the aforeincorporated '671 and '159 patents.

Doping could be done by thermal diffusion, ion implantation, or other semiconductor doping method.

Referring to FIG. 2, there is shown a second semiconductor wafer 200, which may also be referred to as the “substrate wafer” 200. The substrate wafer comprises a body 205, a first surface 203, and a second surface 207. It shall be understood that the thickness of the substrate wafer 200 may vary and is selected based on the pressure range of the targeted pressure sensor and/or on the deemed mechanical robustness of the final device. The substrate wafer 200 may be made of many materials, for example but not limited to silicon. The substrate wafer 200 may be composed of N-type silicon, and in example embodiments, may have a single-crystalline structure. It shall be understood that the substrate wafer 200 may be of many geometries, and more specifically, may have, for example a circular, square, or rectangular cross-section, or may take the form of a commercially available silicon wafer. Further, the substrate wafer 200 may undergo pre-processing steps, for example but not limited to, cleaning, etching, polishing, and/or lapping to provide a smooth surface finish before additional processing.

A second oxide layer 210 may be grown over the first surface 203 of the substrate wafer 200. In an example embodiment, the second oxide layer 210 is thicker than the first oxide layer 115 (depending on the required dielectric capability of the end device and manufacturing process requirements), however in other embodiments, the second oxide layer 210 may be of the same thickness or thinner than the first oxide layer 115. As previously described, one skilled in the art will appreciate that the growth of oxide layers on silicon substrates is well known in the art.

The pattern wafer 100 and the substrate wafer 200 are subsequently bonded together, as illustrated in FIG. 3. Prior to bonding the two wafers together, the pattern wafer 100 may undergo an additional P++ boron doping and/or oxidation process. In some examples, this doping and/or oxidation may take about five minutes or less. This additional boron-doped skin, B₂O₃, may act as an adhesion promoter during the subsequent fusion bonding process between the pattern wafer 100 and the substrate wafer 200. During the fusion bonding process, the pattern wafer 100 is aligned with the substrate wafer 200 having the first oxide layer 115 of the pattern wafer 100 facing the second oxide layer 210 of the substrate wafer 200. The first oxide layer 115 is fusion bonded to the second oxide layer 210, thereby bonding the pattern wafer 100 to the substrate wafer 200. In example methods, the fusion bonding temperature occurs at temperatures between about 800° C. and 1200° C. In other example methods, the fusion bonding temperature occurs at temperatures around 1000° C. for about five to thirty minutes. It shall be understood that the fusion bonding process described herein is not limited to these processing parameters, and may occur at different processing parameters.

After the pattern wafer 100 is fusion bonded to the substrate wafer 200, the base layer 105 of the pattern wafer 100 may be removed. The base layer 105 of the pattern wafer 100 is removed such that the doped layer 110 and the first oxide layer 115 remain bonded to the substrate wafer 200, as illustrated in FIG. 4. There are many techniques that may be used to remove the base layer 105 of the pattern wafer 100. For example, a conductivity selective etchant, such as potassium hydroxide, may be used in accordance with conventional methodology.

Once the base layer 105 of the pattern wafer 100 is removed, a piezoresistive pattern 505 may be defined within the doped layer 110, as illustrated in FIG. 5. By way of further example, a layer of positive or negative exposure photoresist may be deposited over the doped layer 110. This photoresist may be selectively exposed, for example, ultraviolet (UV) radiation exposed, to define a masking pattern. The masking pattern may then be used to selectively etch away portions of the doped layer 110 thereby creating raised and recessed portions that define a piezoresistive pattern. Conventional etching techniques may be used to define the pattern, for example but not limited to, RIE-reactive ion etching, wet etching, ion milling, or other patterning techniques. It shall be understood that the specific piezoresistive pattern may vary depending on the type of SOI device being used and further, a diaphragm member may also be defined within the substrate wafer 200.

SOI devices may employ one or more piezoresistive elements that are mounted or diffused into a bridge pattern of resistors on a thin diaphragm member. The diaphragm member flexes upon application of force thereto and thereby causes stresses on the top surface. These stresses elongate or shorten the piezoresistors, therefore causing them to vary their resistance according to the deflection of the diaphragm. Reference is made to U.S. Pat. No. 4,498,229 entitled “PIEZORESISTIVE TRANSDUCER”, issued on Feb. 12, 1985 to Leslie B. Wilner and to U.S. Pat. No. 4,672,354 entitled “FABRICATION OF DIELECTRICALLY ISOLATED FINE LINE SEMICONDUCTOR TRANSDUCERS AND APPARATUS”, issued on Jun. 9, 1987 to Anthony D. Kurtz et al., assigned to the assignee herein, as illustrative examples of piezoresistive transducer constructions. It shall be understood, however, that many piezoresistive pattern configurations may be defined within the doped layer 110, and that applicability of the present invention is not limited to pressure transducer electronic configurations.

Referring to FIG. 6, there is shown a method flow diagram further detailing an example method of SOI fabrication according to the present invention. First, the pattern wafer and the substrate wafer are prepared independently (steps 605/610). The pattern wafer is oxidized (step 615) to produce a first oxide layer. The pattern wafer is then doped (step 620) to produce a first doped layer. The pattern wafer may be further doped and/or oxidized (step 621). Meanwhile, a surface of the substrate wafer may also be oxidized to produce a second oxide layer 625. The pattern wafer and the substrate wafer are then fusion bonded together such that the oxidized first doped layer of the pattern wafer bonds to the oxidized second semiconductor wafer 630. A base layer of the pattern wafer is then removed using an etching technique to expose the first doped layer 635 and the doped layer is subsequently patterned into a desired piezoresistive pattern 640.

An unanticipated advantage of the various methods described herein is that the substrate and pattern wafers may be prepared prior to patterning, which enables improved process flexibility and efficiency. Specifically, this various methods of fabricating SOI semiconductor devices described herein are more suitable for simpler, large scale fabrication as they provide the flexibility to select the device pattern and type at later stages of fabrication.

It will be apparent to those skilled in the art that modifications and variations may be made in the apparatus and process of the present invention without departing from the spirit or scope of the invention. It is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A method for fabricating a silicon-on-insulator device, comprising: bonding a first semiconductor wafer, having a base layer, a degenerately doped layer disposed on the base layer, and a first unannealed oxidized layer disposed on the doped layer, to a second semiconductor wafer having a second oxidized layer disposed thereon, wherein the degenerately doped layer is doped to a generally uniform depth through the unannealed oxidized layer, and further wherein the bond is between the first oxidized layer and the second oxidized layer; removing the base layer of the first semiconductor wafer to expose the doped layer; and patterning the exposed doped layer to define a piezoresistive pattern on the second doped side of the doped layer.
 2. The method of claim 1, wherein the doped layer is blanket doped.
 3. A method for fabricating a silicon-on-insulator (SOI) device, comprising: oxidizing a surface of a first semiconductor wafer to produce an oxidized layer having an oxidized surface, the first semiconductor wafer having a base layer; degenerately doping the first semiconductor wafer through the oxidized layer to produce a doped layer having a first doped side and a second doped side; bonding the oxidized surface of the first semiconductor wafer to an oxidized second semiconductor wafer; removing the base layer from the first semiconductor wafer, without use of a buried oxide etch stop, to expose the second doped side of the doped layer; and patterning the exposed second doped side of the doped layer to define a piezoresistive pattern on the second doped side of the doped layer.
 4. The method of claim 3, wherein the doped layer is blanket doped.
 5. The method of claim 3, wherein the doped layer is P++ boron doped.
 6. The method of claim 3, wherein the bonding occurs between about 800° C. and 1200° C.
 7. The method of claim 3, further comprising defining a diaphragm member in the oxidized second semiconductor wafer.
 8. The method of claim 3, wherein the oxidized second semiconductor wafer has a thickness that is determined based on a desired pressure range.
 9. The method of claim 3, wherein the oxidized layer has a thickness of about 200 angstroms or less.
 10. The method of claim 3, wherein the oxidized second semiconductor wafer has an oxide layer that is thicker than the oxidized layer of the first semiconductor wafer.
 11. The method of claim 3, wherein the exposed second doped side of the doped layer is patterned using RIE-reactive ion etching techniques.
 12. The method of claim 3, wherein the exposed second doped side of the doped layer is patterned using wet etching techniques.
 13. The method of claim 3, wherein the degenerate doping is done through diffusion.
 14. The method of claim 3, wherein the bonding of the oxidized surface of the first semiconductor wafer to the oxidized second semiconductor layer occurs without annealing the oxidized surface of the first semiconductor wafer.
 15. A method for fabricating a silicon-on-insulator (SOI) device, comprising: oxidizing a surface of a first semiconductor wafer to produce a first oxidized layer having a first oxidized surface, the first semiconductor wafer having a base layer; degenerately doping the first semiconductor wafer through the first oxidized layer to produce a doped layer below the first oxidized layer, the doped layer having a first doped side and a second doped side; doping the first oxidized surface to provide a doped skin; oxidizing a surface of a second semiconductor wafer to produce a second oxidized layer having a second oxidized surface; bonding the first oxidized surface to the second oxidized surface; removing the base layer from the first semiconductor wafer to expose the second doped side of the doped layer; and patterning the exposed second doped side of the doped layer to define a piezoresistive pattern on the second doped side of the doped layer.
 16. The method of claim 15, wherein the first oxidized layer further has a first thickness and wherein the second oxidized surface has a second thickness that is thicker than the first thickness.
 17. The method of claim 15, wherein the exposed second doped side of the doped layer is patterned using RIE-reactive ion etching techniques. 